This invention relates to an improved process and apparatus for forming articles from molten materials including an innovative mold valve gating apparatus and a mold valve gating method useful for injection molding articles of various shapes.
Various valving arrangements have been proposed in the prior art for regulating the flow of heated plastic material from a source of such material to a mold cavity space. In most instances, regulation of the flow of the melt between a hot runner or cold runner nozzle, through a mold gate and into the mold cavity space, is satisfactorily achieved using a valve stem located in the melt channel of the nozzle. The valve stem is actuated by a motive means, typically located in the mold back plate, to open and close the access of the melt to the mold cavity space. This approach has several drawbacks when it has to be applied to a multicavity mold used to form demanding articles, such as blowable preforms, molded using single or multiple materials. One drawback is where the system requires the use of multiple valve stems. In such an arrangement, the individual actuation of the stem creates problems when it comes to ensuring that all the valve stems are opened and closed simultaneously. Second, the valve stem has the tendency to split the flow of molten material, thus creating the so called unacceptable knit lines. Additionally, the actuation of the stems becomes very problematic for multimaterial injection nozzles when at least two materials are injected in the same cavity space. One known approach intended to solve the first two problems is to use a lateral valve gating arrangement comprising a gate orifice. A gate orifice is shown in U.S. Pat. No. 4,108,956 to Lee.
The approach shown in the Lee patent is not very effective. While it solves the first and second aforementioned drawbacks, it does not simplify the mold design and operation. In fact, the approach complicates both mold design and operation. The valving arrangement shown in Lee involves the use of a reciprocally movable slide member having at least one opening. The slide member is interposed between a source of heated plastic material such as a hot runner outlet or injection nozzle and a mold gate. As can be seen in both of these patents, the movable valve carries with it a hot slug of the plastic material being molded as the valve moves from a valve open to a valve closed position. During the cooling step, the slug is solidified. The presence of such a slug in the valve opening requires the apparatus to include some additional and specialized mechanism for removing it from the valve opening. Should the plug not be ejected from the valve opening, it will be brought back to the gate area and injected into the cavity space with the next shot. In many applications, this is not acceptable as it lowers the cosmetic and strength characteristics of the molded article. This aspect becomes even more critical in multimaterial molding where mixing two or more different materials must be avoided.
In the Lee Patent, a movable mechanical ejection unit actuated by pneumatic means (that is as complicated as the similar means used to move a classical valve stem) is positioned on the molding machine to remove the cold slug from the valve opening. Other means, not shown by Lee, must be further used to remove the slugs from the mold. The use of such means becomes very difficult in systems utilizing a multicavity mold, such as the molds having more than sixteen cavities manufactured by the assignee of the instant application. For example, the molds would become very big and heavy if they had to accommodate movable mechanical ejection devices. Further, additional detection means would be needed to make sure that the slug has been indeed ejected from each opening.
A machine, such as that shown in the Lee Patent, is further economically disadvantageous in that the slug is wasted material that could otherwise be used to fabricate molded articles. In addition to the slug removal and waste problems, these systems must face the potential problem of having molten plastic material flow between the surfaces of the valve and the adjacent mold plate and hot runner housing. Should molten plastic material flow between these surfaces and reside therein, operation of the valve and the machine itself could be interrupted if the plastic material solidifies.
The latter valve gating arrangement of Lee has been previously used to control flow of the melt from the machine injection nozzle to a mold as shown in U.S. Pat. No. 3,632,729 to Bielfeldt. Certainly, the valving arrangement shown in the Lee patent was not conceived to be used or adapted for valve gating a multimaterial injection nozzle such as that disclosed in U.S. Pat. Nos. 4,863,665; 5,200,207; 5,143,733; 5,112,212; 4,863,369; 4,808,101; 4,775,308; 4,717,324; 4,701,292; and 4,657,496, all assigned to the assignee of the instant application and all which are hereby incorporated by reference herein. Lee's approach would not be useful in multimaterial molding because it would generate too much waste made of various materials. Further, the need to handle more than one type of slug would become prohibitive both technically and price-wise.
It is also known in the prior art to remove a crystallized gate vestige from a parison (preform) by using reciprocally movable cutting blades. Such a system is shown in U.S. Pat. No. 4,380,423 to Aoki. While the Aoki patent addresses the problem of removing the sprue from the already molded article, it does not address the critical problem of how to open and close off the flow of molten material from a hot runner system to a mold gate without forming the crystallized gate vestige that needs to be cut in a post molding operation. Similar to Lee's slug, the crystallized gate vestige shown by Aoki represents a significant waste of valuable resin, incurring the problems previously mentioned.
There remains a need for a simpler and more efficient mold valve gating arrangement for a multimaterial injection nozzle. There also remains a need for a mold valve gating arrangement for injecting multiple or single materials wherein the flow of molten material from a hot or cold runner system or injection nozzle to a mold gate can be interrupted without incurring waste of the material being molded. There further is a need for a mold valve gating arrangement which does not require the inclusion of a movable and/or a mechanical ejection unit for removing plastic material being molded from the valve.